Investigation of Aztec Wash pluton using 87sr/86sr as a tool to understand mushzone differentiation

Understanding how silicic magma chambers form in the upper crust remains a highly debated issue. While the community has shifted toward views of incrementally emplaced magma chambers, our understanding of how these systems differentiate in the upper crust is still poor. Over the past decade, there has been growing geochronologic evidence suggesting that silicic magma chambers/plutons cool rapidly (<<1 Myrs) from 850 to 550°C but then reside at 550°C to 330°C for 10+ Myrs, far below the haplogranite solidus (725°C at 0.1 GPa). Given that these systems spend relatively long durations at these low temperatures, is there a differentiation process that can occur at these lower temperatures to explain the compositional zoning? The Aztec Wash pluton (AWP), located in Southern Nevada, is part of the Colorado River extensional corridor. Previous work characterized the AWP as consisting of two zones: 1) an overlying homogeneous zone composed of a single composition granite and 2) a heterogeneous zone consisting of repeated sequences of interlayered quartz monzodiorite and quartz monzonite. Here I examine a vertical and horizontal transects crossing lithologic and zonal boundaries to test the hypothesis that a low temperature melt-rock reaction process results in diorite differentiating to granite. Specifically, we measure Sr isotopic ratios in minerals in the quartz monzonite to answer the question of whether they have Sr ratios like those of the quartz monzodiorite. In this study using MC-ICPMS, I present solution-based 87Sr/86Sr analyses of micro-drilled K-feldspars and whole-rock (WR) powders as well as in situ laser-ablation (LA) 87Sr/86Sr analyses of plagioclase. These samples occur along a 10-meter vertical transect of quartz monzodiorite and quartz monzonite rocks that are well characterized by petrographic and x-ray iii mapping analysis across the contact. WR 87Sr/86Sr show an abrupt change in isotopic signatures across the contact: quartz monzodiorites range from 0.7089 to 0.7091 while quartz monzonite ranges from 0.7099 to 0.7100. These isotopic differences would traditionally be interpreted as reflecting two-distinct magma sources. However, micro-analysis of LA plagioclase and micro-drilled plagioclases in the monzonite have 87Sr/86Sri plagioclase values of 0.7096-0.7089, far below the WR value, with some plagioclase matching the whole rock values of the quartz monzodiorite. Generally, K-feldspar in the quartz monzonite contain 87Sr/86Sri that match WR values. These data are consistent with the hypothesis that the quartz monzonite formed from a precursor quartz monzodiorite by the proposed melt-rock reaction process.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Geology

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2023

Author dc:creator

• Velazquez, Mario Gabriel

Contributors dc:contributor

• Lundstrom , Craig C
• Stewart, Michael

• Petrology
• Geochemistry

Statement dc:rights

• Copyright 2023 Mario Velazquez

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/120141

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F120141